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Toroid core to condense magentic flux?

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GhostNull

Mon Jan 25 2010, 02:31AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Hey

I was thinking of using a cut toroid core, as shown in the attachment. This would create a dense magnetic field arcross the barrel. I have found no infomation about this method or people using it. There probaly is some reason this won't work oblivious to me. So tell me tell me your thoughts.

Turkey9

Mon Jan 25 2010, 07:07AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Are you proposing using this to attract the projectile? If so, the field inside a solenoid coil is going to be just as dense as your toroid. The reason a toroid only has a field inside of itself is not because it takes the field from outside and puts it inside. It just has a geometry that cancels out the field outside the coil. So with the same amount of turns and current, a toroid and solenoid will both have the same field strength inside of them. The slotted toroid might work but it wouldn't be practical when the same results can be had much easier.

GhostNull

Mon Jan 25 2010, 10:40AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Turkey9 wrote ...

Are you proposing using this to attract the projectile? If so, the field inside a solenoid coil is going to be just as dense as your toroid. The reason a toroid only has a field inside of itself is not because it takes the field from outside and puts it inside. It just has a geometry that cancels out the field outside the coil. So with the same amount of turns and current, a toroid and solenoid will both have the same field strength inside of them. The slotted toroid might work but it wouldn't be practical when the same results can be had much easier.

Yes I am preposing to use it in a reluctence coil gun. But if, as you say, it cancels out the field outside the coil wouldn't this mean less eddy currents, less stray magnetic field and therefore better efficiency?

Dr. Slack

Mon Jan 25 2010, 03:35PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It's true that a core such as you have drawn would increase the flux in the gap over what you might expect with a small excitation current. It may sort of work in a coil gun.

However, that's not usually the problem with a coil gun.

Generally you can get enough energy in reasonable size/cost capacitors to saturate any magnetic material using a simple air-cored solenoid, so it's not actually needed. People do try to enhance the field by putting some iron in the return patth round the coils, but there's debate about whether even that's worth the effort.

You also need to keep the inductance low to avoid suck-back after the armature has passed the middle position. The core would give you a relatively high inductance.

radiotech

Mon Jan 25 2010, 04:48PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Your diagram and description made me think of this magnet design.

GhostNull

Wed Jan 27 2010, 07:54AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Okay, thanks guys.

So in a summary this design would be feasible but the solenoid design would achevie just as good results with less trouble

Barry

Wed Jan 27 2010, 05:32PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

GhostNull wrote ...

So in a summary this design would be feasible but the solenoid design would achieve just as good results with less trouble

Actually, I really like this idea. My experience has been that external iron makes dramatic improvements, although input power level should be chosen to avoid saturation. If the goal is maximum energy conversion efficiency, then I'm convinced that some kind of iron is essential to control magnetic flux. And this configuration looks better than whacking up E-leaves from a transformer core like I've done so far.

The toroid holds promise for better guidance of magnetic flux path. I However, I think there are limitations.

Some toroidal advantages...

no square bends means less loss to fringing and stray fields

reasonably high magnetic saturation level compared to alternative materials

high saliency ratio means high efficiency

powdered iron toroid means no eddy currents

by choosing a toroid with a larger cross-sectional area you can operate at higher power (but it's still limited by projectile saturation)

Some toroidal disadvantages...

harder to build, cutting and forming brittle toroidal cores can be tricky

pulse can put large mechanical force on brittle core and break it

easy to saturate core means input power should be limited or at least carefully chosen

limited input power means multistage design would be required to reach higher velocity

I'd suggest improvement by refining the barrel area. A guiding principle of all magnetic machines is to minimize gaps using close tolerance. This reduces fringing and air path when the projectile is fully enclosed. We want the highest saliency ratio in the machine, ie, the largest ratio of maximum inductance to minimum inductance. So the sketch might end up more like this diagram, where the toroid touches the barrel instead of a straight radial cut.

Cheers, Barry
We use vaseline during sex. It works great. Just put it on the doorknob and it keeps the kids out.

GhostNull

Thu Jan 28 2010, 11:01AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

With creating the slotted toroid you could use the core from a power toroid/toriodial transformer some use a laminated iron core and aren't brittle at all. You could even you the coils all ready wound on it but then they won't be designed for the capacitors and the other stuff in your coilgun and there would be some internal eddy current as opposed to the powder core.

TheMerovingian

Thu Jan 28 2010, 01:40PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Using a light toroidal core will waste most of the power in the air since the toroid will saturate, using a heavy toroidal core will raise the inductance and slowen the pulse edge, it may work for low velocity machines with low currents but i doubt that it will give advanmtages in a classical coilgun setup

GhostNull

Fri Jan 29 2010, 10:05AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

what do you mean by light and heavy toroidal core

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